1. Field of the Invention
This invention relates generally to dilation catheters. More particularly, the invention relates to intravascular angioplasty catheter balloons and a method of manufacturing the same.
2. Description of the Related Art
Angioplasty is a procedure by which stenotic lesions (atheromatous deposits), found in cases of atherosclerosis. During angioplasty, a guidewire is inserted into the cardiovascular system, generally via the femoral artery under local anesthesia. The guidewire is advanced through the patient""s vasculature to the site of the stenosis (stenotic lesion). Placement of the guidewire may be aided by way of fluoroscopic observation. A dilatation catheter, having a guidewire lumen and distensible balloon portion, is then advanced through the vasculature until the balloon portion, at the distal end of the catheter, traverses or crosses a stenotic lesion. The artery is narrowed in the area of the stenotic lesion due to the atheromatous deposits occupying arterial space at the walls of the artery. Once placed, the balloon portion of the catheter is inflated, generally with a fluid, to compress the atheromatous deposits against the walls of the artery. This compression dilates the lumen of the artery leaving an unblocked arterial passage once the guidewire and catheter are removed.
Looking back to where the uninflated balloon encounters the stenosis, it must first cross at least a portion thereof in order to reach its distal-most destination. Therefore, a flexible, low profile balloon is preferable. In particular, the ends of the uninflated balloon should taper smoothly and lay low so that the balloon can be threaded into tight passages. It is preferable that the thickness of the balloon material be substantially constant from a working length throughout each taper. In the present context, a thick wall is at least approximately 0.002xe2x80x3 in thickness while a thin wall is approximately 0.001xe2x80x3 in thickness.
Unfortunately, current production methods yield a balloon with stiff and bulky tapers. These limitations are related to the behavior of the balloon material during manufacture, where a piece of polymer tubing is stretched to make the balloon. The balloon is made (xe2x80x9cblownxe2x80x9d) by placing a segment of polymeric tubing in a mold, heating it to a near-molten state, and pressurizing the tubing until it fills the mold. The tubing within the mold forms the balloon. The mold is shaped such that the balloon is comprised of a working length with a taper at each end thereof. Each taper joins an unexpanded segment of tubing outside of the mold, referred to here as a shaft. Because the tapers expand less than the working length, they remain stiffer and bulkier. A thin-walled taper would be more desirable.
One approach to thinning the wall of the taper is a process called xe2x80x9cpre-neckingxe2x80x9d in which the segment of tubing that will become the taper is first softened by heating and then subjected to a force which forms a narrowed segment in the tubing, referred to here as a neck. The objective of pre-necking is to form the taper from this neck. As the balloon is blown, the neck expands to form a taper having thinner walls than a taper blown from un-necked tubing. The thin taper terminates at a thin shaft. However, the problem of thick, stiff tapers still remains to a certain extent because the pre-necking is performed in a solid or semi-molten state in which the strain applied to the tubing induces crystallization. In effect, the molecular strands of the polymer become aligned parallel to the load inducing the strain. Once aligned in this manner, the polymer resists further distension. Thus, due to pre-necking, we have exchanged a thicker taper for a somewhat thinner taper which nonetheless remains less expansive than the reminder of the balloon. The remainder of the balloon, which is intended for contacting the wall of a body lumen such as during an angioplasty, is often referred to as the working distance or the working length. In the case of pre-necked balloons we end up with a thin taper which is less expansive than the working length.
What is needed, therefore, is an angioplasty balloon having a thin taper terminating at a thin shaft. It is desirable that the thin taper have a wall of substantially equivalent thickness to a wall of the working length.
It is an object of the present invention to provide an angioplasty balloon having a taper thickness substantially equivalent to a working length thickness.
It is an object of the present invention to provide an angioplasty balloon having a thin shaft.
It is an object of the present invention to provide an angioplasty balloon having a wall thickness no greater than 0.002xe2x80x3, and in one embodiment between 0.0005xe2x80x3 and 0.002xe2x80x3.
It is an object of the present invention to provide a slug capable of being molded into an angioplasty balloon having a taper thickness substantially equivalent to a working length thickness.
It is an object of the present invention to provide a slug comprising a polymeric inner tube within a shortened polymeric outer tube.
It is an object of the present invention to provide a method of manufacturing an angioplasty balloon having a taper thickness substantially equivalent to a working length thickness.
In accordance with these objectives an angioplasty balloon 40 is provided having a taper wall thickness 76 substantially equivalent to a working length wall thickness 60. The angioplasty balloon 40 is manufactured from a slug 100 having an inner tube 106 within a shortened outer tube 102. The shortened outer tube 102 is fused to the inner tube 106 within a mold until an angioplasty balloon 40 has formed. The working length 44 of the angioplasty balloon 40 has formed from the shortened outer tube 102 while the inner tube 106 forms a taper (48, 50) at each end of the working length 44. Each taper (48, 50) terminates in a shaft (42, 46). The working length 44, taper (48, 50), and shaft (42, 46) each have substantially equivalent wall thicknesses (60, 66, 76).